The role of the sex hormones in the differentiation and development of the neonatal gonadotrophic axis will be studied in long-term, organotypic cultures of rodent hypothalamus plus or minus pituitary. The cytological parameters of development will be defined in living and stained preparations at the light microscopic level. The following aspects will be analyzed: 1. Comparison of the morphogenesis of the genetic male plus female hypothalamus cultured with and without pituitary; 2. The modifying effects of estrogen and androgen added during selected developmental stages to document changes associated with the "critical period" and with maturation; 3. The morphogenesis of the neurosecretory process for the gonadotrophic releasing factors (Gn-RF) to identify and localize the neurons involved and to see if they are the same as the sex steroid sensitive ones. Corroborative information will be obtained by radio-immunoassay of the nutrient medium for the gonadotrophic RF and hormones and by immunoenzyme and autoradiographic methods for their cellular localization. This unique model can provide new information heretofore unobtainable by other methods on the developmental aspects of hypothalamic regulation of gonadotrophin secretion and the way by which the sex hormones modify these regulatory processes. These aspects are critical to an understanding of sexual differentiation of the brain, psychosexual behavior and to the reproductive processes proper. BIBLIOGRAPHIC REFERENCES: Araki, S., Toran-Allerand, C.D., Ferin, M,, and Vande Wiele, R.L., Immunoreactive gonadotropin-releasing hormone (Gn-RH) during maturation in the rat: Ontology of regional hypothalamic differences. "Endocrinology 97," 693-697 (1975). Toran-Allerand, C.D., Development of the hypothalamic-pituitary-thyroid axis: Ontogenetic aspects of the neuroendocrine unit. In "The Thyroid," 4th Ed., Sidney C. Werner and Sidney Ingbar, eds., Harper & Row, N.Y. 1975. In press.